1. Field of the Invention
The invention is generally aimed at a process for the preparation of melt spun, melt-colored, fibers. In particular, the invention relates to a process to form melt-colored fibers, from blends of at least one fiber-forming polyamide with at least one polyester, that exhibit improved color and aesthetics in comparison with equivalent melt-colored fibers manufactured using polyamide alone. In addition, the melt-colored fibers also exhibit improved dimensional stability when the fibers are exposed to changes in temperature and/or humidity.
2. Description of Related Art
Coloration of fibers has a long history, and the science of dyeing, initially of natural fibers such as flax, cotton and wool, has been under continuous development since Neolithic times. The appearance of man-made fibers, (e.g., cellulosics, acrylics, polyamides and polyesters), stimulated further developments in dyeing, and this method of coloring of fibers and articles made therefrom continues to be the most-practised technique for the production of colored fiber-based articles of manufacture.
In the case of fibers based on the more recent polymers such as polyamides and polyesters, which are spun from the melt, there exists an alternative method for coloration, i.e., addition of the colorant species into the melt and direct extrusion of colored fibers. While such a process may be carried out with dyes, it is more often carried out with pigments. Notwithstanding this fact, the process is popularly known throughout the industry as “solution dyeing”. The major difference between dyes and pigments is that, under prevailing processing conditions, pigments are virtually insoluble in polymers, whereas dyes are soluble, (see definitions in German Standards DIN 55943, 55944 and 55949, incorporated herein by reference).
As the technique of melt-pigmentation has been developed, it has been demonstrated that fibers made in this way can exhibit certain advantages over those made by post-spinning dyeing of fibers. Such advantages include improved resistance to degradation and fading in sunlight; lower susceptibility to fading and/or yellowing by polluting gases in the atmosphere, such as ozone and nitrogen oxides; improved resistance to chemicals, either in dry-cleaning processes or encountered in accidental spillages; less leaching or fading of color during laundering or cleaning processes involving water and detergents; no need for post-spinning industrial processes to color the products or to fix the color in place.
However, melt-pigmentation is also considered to have some disadvantages in terms of the color and appearance obtained in the final fiber. The fibers are generally regarded by those skilled in the art to exhibit degrees of lustre and low brightness that can render the said fibers unsuitable in certain applications.
The color change resulting from the addition of pigments to polymers is based on the wavelength-dependent absorption and scattering of light, with the appearance and color of the final product being a combination of these two factors as described in the Kubelka-Munk theory. A description of this theory, along with the general concepts of color and its measurement, may be found in “Colour Physics for Industry”, Roderick McDonald, (Ed.), The Society of Dyers and Colourists, Bradford, UK, 2nd Edition, (1997). Dyes can only absorb light and not scatter it, since the physical prerequisite for scattering—a certain minimum particle size—does not exist in the case of dyes in molecular solution; these colors are therefor transparent. Insofar as the transparency may be said to be attributable to the dye, complete absorption of light will result in black shades, selected absorption will result in colored shades.
The optical effect of pigments may in the same way be based on light absorption. If, however, the refractive index of the pigment differs appreciably from that of the polymer which is almost invariably the case, and if a specific particle size range is present, scattering takes place. Under these conditions, the initially transparent polymer becomes white and opaque, or, if selective absorption takes place at the same time, colored and opaque.
No scattering occurs when the particle sizes are very small, and none or very little occurs if they are very large. With all colored pigments that selectively absorb, the shade and strength of the final color is thus influenced by particle size. The transparency and thickness of the colored substrate may additionally affect the color strength. While some pigments are available in so-called transparent grades, e.g., red and yellow iron oxides, a complete color range across the spectrum is not readily available. Many such ultra-low particle size colorants are expensive, and difficult to maintain at high dispersion when compounded into a polymer matrix. It is also known that very low particle size additives in polymer melts can produce a profound effect on the rheological properties of said melt, resulting in formulations which are difficult to spin using standard equipment and procedures. Use of large particle size colorants is not a viable option either, as such additives will result in the blocking of spinneret orifices, pressure problems with filtration systems, and will lead to unacceptable levels of filament breaks in fiber production.
In any case, a large number of melt-pigmented fiber products are required to be opaque, and the problem lies in producing colored fibers with levels of color brightness close to those of dyed products. Note that a fundamental difference between dyeing and pigmenting of fibers is that, while dyes, or their mixtures, are either colored or absorb all wavelengths of light, (i.e., give black shades), pigmentation introduces an extra variable in that white pigments are readily available, whereas there is no such species as a “white dye”, nor can any combination of dyes result in a white fiber.
Another problem with particulate colorants in a polymeric melt-spun fiber is the phenomenon of dichroism, or optical anisotropy. Pigment particles are not necessarily isotropic in shape, and indeed may be needle-shaped, rod-shaped, or platelets. They may thus become oriented in a preferred direction due to the forces they encounter during processing of the melt and of the fiber. The apparent color then depends on the direction of observation. The origin of this phenomenon is to be found in the fact that certain pigments crystallise in crystal systems of low symmetry, resulting in directionally dependant physical properties. As far as the coloristic properties are concerned, this means that the absorption and scattering constants differ in the various principal crystallographic axes, i.e. such crystals are optically anisotropic.
With regard to the final fiber, as opposed to the above comments on the pigments themselves, the appearance of a sample thereof can vary depending on the angle of illumination and/or observation. Fiber samples are normally prepared for color and appearance testing by carefully wrapping the fiber or yarn sample, under conditions of uniform tension and consistent positioning of the said fibers, around a flat “card”, and assessing the color properties, and, more importantly, any differences between said properties and those of the desired sample or data, under standard conditions of illumination and observation. This may be carried out visually, but more usually is carried out using instrumentation. Methods and apparatus for carrying out the analysis of color and appearance in this manner are well known to those skilled in the art, and are not discussed in detail herein.
During such examinations, there are two additional effects that might be observed if the sample is illuminated or observed at a number of different angles. An example of multi-angle appearance testing of materials is reported in U.S. Pat. No. 4,479,718, assigned to DuPont. The first possible effect is that the total amount of light reflected from the sample, per unit area, may change. The second possible effect is that the ratio of the various reflected and scattered wavelengths may change, resulting a change in the measured or perceived color. Unless such effects are specifically desired for particular aesthetic reasons in a final article of manufacture, the appearance of either may result in rejection of the fiber by the prospective customer. Eradication or reduction of such effects is thus important in obtaining first quality product.
The basic concept of blending of at least two polymers in a melt in order to manufacture articles is well known in the art, and this technique has been applied in a number of cases for the manufacture of fibers. In the majority of cases, polymer pairs are incompatible, i.e., the minor component of the blend forms a dispersed, discrete, phase within a matrix of the major component.
Where the incompatibility between the polymers is pronounced, the physical properties of the blend are liable to be poor, there being little or no adhesion between the matrix and the dispersed phase. A widely practised remedy to this problem is the use of so-called compatibiser additives. Such additives may function in a variety of ways, including acting as surfactants to reduce the surface tension between the two phases of the blend, chemically reacting with both phases to “tie” them together, or physically interacting with both phases to bind them together. Particular compatibilisers may function in more than one way. The use of such additives in polymer blends is well known to those skilled in the art. A detailed discussion of this aspect of polymer technology may be found in “Compatibilising agents—Structure and function in polyblends” by N. G. Gaylord—Journal of Macromolecular Science Chemistry, A26, 1211-1229 (1989).
Fibers produced by melt-blending using polyamide as major component and polyester as minor component are known in the art, and have been manufactured with a variety of purposes in mind.
One area in which this method has been successfully applied is in the field of reinforcing yarns for pneumatic tyres, (“tyre-cord”). Examples of such prior art include U.S. Pat. Nos. 3,369,057 and 3,470,686, assigned to Allied Chemical Corporation. In those patents, fibers are spun which comprise a dispersed phase of polyester, such as poly(ethylene terephthalate) in polyamide, such as polyamide 6. The use of such materials in a tyre-cord is claimed to provide tyres with less susceptibility to “flat-spotting” than have tyres reinforced with cords made from polyamide alone.
In U.S. Pat. No. 3,549,741, assigned to Allied Chemical Corporation, there is the disclosure of the production of carpet fiber from melt blends of polyamide and polyester, but the invention states quite clearly that such blends are require to be processed using non-standard equipment, and under different conditions to those normally used to manufacture carpet fibers from polyamide alone.
In U.S. Pat. No. 6,090,494, issued to DuPont, and in corresponding PCT application WO99/46436, it is asserted that “shaped articles”, including fibers and yarns, may be produced by melt-blending polyamide with one or more pigments in a suitable carrier, and also with 0.5 to 9 weight % of a polyester. It is stated in the patent that the claimed fibers may be spun under standard polyamide processing conditions. While it is noted that the practise of this invention allows inclusion in the blend of certain pigments which cause physical property deterioration in the final fiber if used in polyamide alone, it is nowhere disclosed that any enhancement of color brightness or fiber aesthetics will result from the practise of the invention.
Compatibilised blends of polyamide and polyester have also been disclosed for use in the melt-spinning of fibers. Examples include:
U.S. Pat. No. 3,378,056, (Firestone Tire & Rubber Co.), describes the manufacture of tyre-cord using a blend of polyester and polyamide compatibilised by the addition of poly(hexamethylene isophthalate).
U.S. Pat. No. 4,150,674, (Monsanto), discloses the use of a lactam-polyol-polyacyllactam terpolymer as compatibilising agent in blends of polyamide and polyester for use in the manufacture of non-woven fabrics.
U.S. Pat. No. 4,417,031, (Allied Corporation), is concerned with the use of reactive phosphite species, e.g., tributyl or triphenyl phosphite, as compatibilisation agents in blends of polyamide and polyester for use in the manufacture of tyre-cord.
U.S. Pat. No. 4,963,311, (Allied-Signal), describes a process similar to that in U.S. Pat. No. 4,417,031 discussed above, in which a polyester reacted with phosphite is itself used as a compatibiliser in a blend of polyamide and unreacted polyester. Also aimed at production of improved tyre-cord.
U.S. Pat. No. 5,055,509, (Allied-Signal), claims the use of phosphoryl azides as reactive compatibilisers in polyamide/polyester blends. Once again, this patent is aimed at the manufacture of improved tyre-cord.
U.S. Pat. No. 5,270,401, (DSM NV), describes a melt-spinnable blend consisting of polyamide and polyester, where the two phases are enhanced in compatability by adding both a copolyester which contains up to 50 mol % of an aliphatic dimer fatty acid, and an amine or acid grafted polymer.
None of these patents directed to compatibilised blends of polyamide and polyester note any changes to, or enhancements of, color or appearance of the fibers made therefrom.
A need thus continues to exist in the industry for a simple method to provide pigmented articles, especially melt-spun fibers, with improved color and appearance, whilst still using standard grades of pigment currently known to those skilled in the art to be suitable for this purpose, and equipment and techniques known to produce melt-pigmented fibers of the required properties for use in manufactured articles such as fabrics, textiles, carpets, threads etc.
One attempt which has been made to achieve this end is described in U.S. Pat. No. 5,674,948, assigned to DSM NV. This patent claims that end-capping of the amino end-groups of polyamides results in improvements to the brightness of compositions colored with organic pigments which are capable of themselves reacting with such amino end-groups. N-acetyl lactam is noted as a particularly preferred end-capper. This approach does not, however, fully solve the problem as the effect is limited to a particular set of organic pigments. The levels of additive end-capper required to achieve a sufficient reduction in the level of free amine end-groups to produce a noticeable effect are also quite high. Finally, the use of this approach requires a prior knowledge of the amine end-group level in the polyamide, (a non-trivial analytical task), and requires that the additive end-capper level be varied from polyamide to polyamide, or even from batch to batch of the same polyamide, to account for differing levels of amine end-groups therein.
When objects made from polyamides, such as fibers, yarns, and products produced from fibers and yarns such as fabrics, carpets and other floorcoverings are subjected to different temperature and humidity environments, changes in the physical dimensions of such objects can occur, including but not limited to, shrinkage, warping, distortion, bowing or curling of the object. These changes in physical dimensions can be objectionable for certain applications of the fibers and yarns.
The present inventors have discovered the surprising fact that melt-pigmented blends of one or more fiber-forming polyamides and one or more thermoplastic polyesters, where the thermoplastic polyester is the minor phase, and which optionally contain suitable compatibilising additives for the two polymer types, may be melt-spun into fibers which exhibit improved color and appearance over similarly colored fibers based on the fiber-forming polyamide alone. The improvement in appearance between two samples can be measured as color strength. In addition, the fibers also have improved dimensional stability.